Method and system for determining the buffer action of a battery

ABSTRACT

A method for determining the buffering effect of the battery ( 2 ) for providing a voltage (U) for a power supply system ( 4 ), in particular for a vehicle, is provided in order to identify serviceability of a battery as easily and reliably as possible, in which method any voltage change and any current change are detected cyclically, the dynamic internal resistance of the battery is determined on the basis of the quotient of the voltage change and the current change, the specific dynamic internal resistance is monitored for exceeding an extreme value which can be predetermined, and a statement of the buffering effect of the battery is made and is output. In this case, the buffering effect of the battery is better, the smaller the quotient of the voltage change and current change.

[0001] The invention relates to a method for determining the bufferingeffect of a battery for providing a voltage, in particular for avehicle. The invention also relates to an arrangement for determiningthe buffering affect.

[0002] Safety-relevant functions, such as electrical brakes, are beingincreasingly operated electrically in vehicles. In order to ensure thatthe serviceability of such safety-relevant components is guaranteed,they must be supplied with electrical power all the time. In order toachieve this, it is known for two energy sources to be provided in avehicle, namely a battery and a generator.

[0003] Since the life of a conventional lead-acid battery is generallyshorter than the life of a vehicle, it is possible for only one of thetwo energy sources to still be serviceable. This situation, in which abattery is discharged or defective and is thus unserviceable, can bedamaged or to overloading of the second energy source, the generator.This can lead to the supply to the loads no longer being ensured.Furthermore, the vehicle may change to a safety-critical state. For thisreason, measures are required which on the one hand warn the driver andon the other hand also keep the vehicle power supply system, and hencethe vehicle, in an operationally safe state for as long as possible. Inparticular, it is necessary to ensure that loads which are relevant foroperation and in particular those which are relevant to safety, such asengine electronics, electrical brakes, are not switched off.

[0004] In addition to ensuring that as little current as possible isdrawn, a minimum voltage level must be maintained in order to guaranteethe operability of safety-relevant devices such as these. This cannormally be ensured by means of the generator. However, if the batteryhas been deep-discharged or is defective in some other way, then it canno longer provide a buffering effect. This may possibly lead to thegenerator being de-energized by a load change during operation and thevehicle power supply system voltage suddenly collapsing, so that it isno longer possible to ensure that the safety relevant devices areoperated.

[0005] A method for determining the buffering effect of a battery forproviding a voltage for a power supply system for a vehicle is knownfrom DE 199 44 517 A1. In this case, the voltage ripple is detected anddetermined, and the maintenance of a limit value which can bepredetermined is monitored using a monitoring unit.

[0006] Furthermore, a method for testing the quality of a battery isknown from Japanese Laid-Open Specification JP 03-249 582 A. In thiscase, a constant AC source is used as a load for the battery to betested, and the voltage level which then occurs is determined. Thequality of the battery is then assessed on the basis of the resultantvoltage levels. However, owing to the use of the constant AC source, anarrangement such as this cannot be used in a vehicle. Furthermore, theuse of the constant AC source represents an extra signal being fed in,and this is complex.

[0007] The invention is thus based on the object of specifying a methodfor determining the buffering effect of a battery, which allows theserviceability of a battery to be identified as easily and reliably aspossible. A further aim is to specify a particularly simple arrangementfor determining the buffering effect of the battery.

[0008] According to the invention, this object is achieved by a methodhaving the features of claim 1, and by an apparatus having the featuresof claim 15. Advantageous developments of the invention are specified inthe dependent claims.

[0009] The invention is in this case based on the idea that, when thebattery has a negligible buffering effect, this leads to a change in thefilter characteristics of the battery for the vehicle power supplysystem. According to the invention, the dynamic internal resistance ofthe battery is determined for this purpose, on the basis of the quotientof any voltage change and any current change. The dynamic internalresistance is preferably monitored for a minimum value which can bepredetermined. This makes it possible to make a statement about thebuffering effect of the battery by monitoring the dynamic internalresistance for exceeding a minimum value. The smaller the quotient ofthe voltage change and the current change, the better is the bufferingeffect of the battery. The dynamic internal resistance is in this casepreferably determined when a significant current or voltage changeoccurs, for example when a change of more than 1 V or 5 A occurs.

[0010] In order to make a statement about the buffering effect of thebattery even when small current and voltage changes occur, the internalresistance is preferably weighted by means of a weighting factor. Inparticular, this results in continuous monitoring of the dynamicinternal resistance, and hence the continuous identification of thebuffering effect. By way of example, the power is used as a weightingfactor, based on the following expression P=ΔU×ΔI. Depending on thenature and the embodiment, the weighting factor can be taken intoaccount as a linear function or square function when determining thedynamic internal resistance.

[0011] A warning message is advantageously output when the limit value,that is to say the minimum value, is exceeded. This ensures that awarning message is output to the driver, for example in the form of anindicator light on the dashboard, when the buffering effect of thebattery is no longer sufficient for emergency or safety operation or forshort-term severe overloading.

[0012] For this purpose, the apparatus according to the invention has,inter alia, a monitoring unit for determining the dynamic internalresistance of the battery on the basis of the quotient of any voltagechange and any current change.

[0013] This and further objects, features and advantages of the presentinvention will become clear from the following detailed description ofpreferred exemplary embodiments of the invention in conjunction with thedrawing.

[0014] Exemplary embodiments of the invention will be explained in moredetail with reference to a drawing, in which:

[0015]FIG. 1 shows, schematically, an arrangement for determining thebuffering effect of a battery, having a sensor unit and a monitoringunit,

[0016]FIG. 2 shows, schematically, the sensor unit,

[0017] FIGS. 3 to 4 show diagrams of a current/voltage characteristicfor a battery, and

[0018] FIGS. 5A-5B to 8A-8B and 9A-9C to 12A-12C show diagrams withfunctional profiles of the voltage and current for various exemplaryembodiments.

[0019] Parts which correspond to one another are provided with the samereference symbols in all the figures.

[0020]FIG. 1 shows an arrangement 1 for determining the buffering effectof a battery 2 for providing a voltage U for a power supply system 4, inparticular for a vehicle which is not shown in any more detail. Thevehicle power supply system 4 is supplied with voltage U from thebattery 2. The vehicle power supply system 4 can be supplied withvoltage U from the vehicle generator 5 as a second energy source. Inaddition, a back-up battery bat2 (not shown) can be arranged in parallelwith the (vehicle power supply system) battery 2, and can be connectedin order to compensate for instabilities in the vehicle power supplysystem and for temporary charging when required.

[0021] The arrangement 1 has a means 6 for detecting and determining thevoltage U and/or the voltage ripple UW (referred to for short in thefollowing text as a voltage sensor 6), and a monitoring unit 8 formonitoring the voltage U and/or the voltage ripple UW for maintenance ofan extreme value G_(U), G_(UW) which can be predetermined. In addition,the arrangement 1 has a means 10 for detecting and determining thecurrent I and/or the current ripple IW (referred to for short as acurrent sensor 10 in the following text). The characteristic variablesfor the vehicle power supply system 4, such as the voltage U, thevoltage ripple UW, the current I and/or the current ripple IW, aremonitored by means of the monitoring unit 8 for compliance with anextreme value G_(U), G_(UW), G_(I) or G_(IW), which can bepredetermined, in order to determine the buffering effect of the battery2. The monitoring is applied to overshooting and/or undershooting of thepredetermined extreme value G_(U), G_(UW), G_(I) or G_(IW). As thebuffering effect of the battery 2 decreases, the voltage ripple UWincreases, and the current ripple IW decreases. The voltage ripple UW iscaused in particular by switching on loads, for example the generator orthe ignition system.

[0022] Thus, the voltage ripple UW is monitored in one exemplaryembodiment. When monitoring the current ripple IW, the reciprocalbehavior to that of the voltage ripple is analyzed. If the respectivepredetermined extreme value G_(UW) or G_(IW) is overshot or undershotduring the monitoring of the voltage ripple UW and/or of the currentripple IW, then this is an indication that the buffering effect of thebattery 2 is inadequate.

[0023] The monitoring unit 8 may in this case be in the form of hardwarecircuitry or software. By way of example, FIG. 2 shows a hardwarecircuitry version of the monitoring unit 8 for monitoring the voltageripple UW for compliance with the associated extreme value G_(UW). Inthis case, the monitoring unit 8 has a filter 12, in particular abandpass filter. The filter 12 filters low frequency components, inparticular the DC voltage, and high frequency components out of thevoltage signal U. Depending on the nature and embodiment of the filter12, this is a filter whose mid-frequency is derived from the rotationalspeed of the engine or of the generator 5. The filter 12 is followed bya rectifier 14 and a comparator 16. The oscillation which is filteredout of the voltage signal U by means of the filter 12 is converted bythe rectifier 14 to a DC voltage, and is compared by the comparator 16with the predetermined extreme value G_(UW). In other words: theamplitude of the voltage ripple UW is monitored by means of thepredetermined extreme value G_(UW) for a maximum value. If the DCvoltage downstream from the rectifier 14 is greater than the maximumvalue, then the buffering effect of the battery 2 is too low, and awarning message M is output, for example in visual form as “chargebattery” or “replace battery”, or audibly.

[0024] By analogy with the hardware circuitry monitoring unit 8 for thevoltage ripple UW, the current ripple IW is measured by tapping off thevoltage U across a shunt resistance which is connected in series withthe battery 2. In this case, the amplitude of the current ripple IW ismonitored for undershooting a predetermined minimum value. In additionto determining the ripples UW and/or IW, the buffering effect can alsobe assessed by using the response of the voltage U and current I, inparticular when switching on a load, for compliance with associatedextreme values G_(U) and G_(I). A comparator 16 based on hardwarecircuitry is likewise provided for this purpose.

[0025] In the exemplary embodiment according to the invention, themonitoring unit 8 may, for example, be in the form of a computer unit.The monitoring unit 8 is in this case used for monitoring and assessingthe vehicle power supply system variables, that is to say the voltage U,the voltage ripple UW, the current I and/or the current ripple IW. Byway of example, the computer unit is a microcontroller or some otherdata processor unit. In order to take account of relationships betweenthe ripples, such as the current ripple IW and/or the voltage ripple UW,the monitoring unit 8 has families of characteristics, for example forrelationships between the voltage ripple UW and the temperature, thegenerator DC voltage, the generator current and the battery current I.These families of characteristics are stored, for example, in the formof curves or tables.

[0026] In the preferred exemplary embodiment of the invention, inaddition to the measurements of the voltage U and current I as describedabove, the dynamic internal resistance Ri of the battery 2 is determinedusing the monitoring unit 8, and is used to identify the bufferingeffect. Both the current change ΔI and the voltage change AU aredetermined in this case. The quotient of the voltage change AU and ofthe current change ΔI is used to determine the dynamic internalresistance Ri and to monitor for compliance with a predeterminedassociated extreme value G_(R), in particular for a minimum value beingexceeded. If the minimum value is exceeded, then the buffering effect isno longer adequate. This means that, the smaller the quotient of thevoltage change ΔU and the current change ΔI, the better is the bufferingeffect of the battery 2. The dynamic internal resistance Ri ispreferably determined when a minimum change occurs in the current I andvoltage U, for example when a current change ΔI of at least 5 A and avoltage change ΔU of at least 1 V occur.

[0027] The method according to the invention for determining thebuffering effect of the battery 2 will now be described in more detailin the following text.

[0028] In the method according to the invention, a new value pair for avoltage U and a current I from the battery 2 is determined cyclically,for example every 4 ms. This results in the voltage change AU and thecurrent change ΔI. The monitoring device calculates filtered values fromthis value pair. The filter is, for example, a first-order low-passfilter or a VZ1 element. In this case, a high-speed filter is in eachcase preferably formed, for example with T=8 ms for the voltage U andcurrent I. If the voltage is additionally filtered via a slow filter,for example in which T=30 ms, the voltage and current profiles can bechecked for plausibility, and voltage and current flanks can be selectedin advance.

[0029] These filtered voltage and current values are then searched bythe monitoring device for relative minima and maxima. The dynamicinternal resistance Ri is determined after each identified extremevalue, that is to say relative minimum or maximum. However, this processof determining the dynamic internal resistance Ri is carried out onlysubject to the following boundary conditions:

[0030] Before the identified extreme value, start values resulting froman opposite extreme value or the end of a maximum waiting time for thenext extreme value will have already been stored.

[0031] The sudden changes in the battery voltage from the high-speed andslow filters have the same mathematical sign. Sudden changes such as thefalling flanks of a load dump or load shedding, when the currentdecreases sharply, but the voltage does not change to the same extent,are thus ignored.

[0032] The dynamic change to the battery voltage must have a specificminimum magnitude. In the case of very large sudden voltage changes andsudden changes which are completely in the discharge area, it issufficient to check that the slow voltage change is not greater than therapid change.

[0033] In the case of sudden changes in which the current starts or endsin the discharge area, the high-speed sudden voltage change must be15-30% greater than the slow sudden voltage change.

[0034] Sudden changes which are completely in the charging area musthave the greatest dynamic response. The high-speed voltage change mustbe 25-50% greater than the slow voltage change.

[0035] When the generator is running, this can be identified from thefact that the sudden voltage change from the high-speed filter must begreater by a specific factor than that of the slow filter (for example25%). For very large sudden voltage changes (>2 V) or when the generatoris inactive, it is sufficient to check that the difference from thehigh-speed filter is at least not less than that from the slow filter.

[0036] The mathematical sign of the sudden voltage change and suddencurrent change must be the same (dU*dI>0), or the current must be belowthe tolerance limit.

[0037] If these boundary conditions are satisfied, the dynamic internalresistance Ri is calculated.

[0038] Before calculating the dynamic internal resistance Ri, a check iscarried out to ensure that no division by zero is being carried out orthat a negative value has been determined for Ri. If the magnitude ofthe current is below the tolerance limit, the mathematical sign of thevoltage and a minimum value are transferred.

[0039] The significance of the identified flank is determined on thebasis of the measurement error to be expected.

[0040] Based on the formula:${Ri} = \frac{{dU} \pm {2 \times {fU}}}{{dI} \pm {2 \times {fI}}}$

[0041] and taking account of the maximum resistance (measured voltagehigh, measured current small):${significance} = {{abs}\left( \frac{1}{1 - \frac{{{dU} \times {dI}} + {2 \times {fU} \times {dI}}}{{{dU} \times {dI}} - {2 \times {fI} \times {dU}}}} \right)}$

[0042] where fU and fI are the errors to be expected from the voltageand current measurements respectively (for example noise, quantizationerror from the A/D converter, . . . ).

[0043] The significance is thus derived from the relative error in theresistance determination (relative error=0 . . . 1).

[0044] The limit value analysis results for very large sudden voltageand current changes with a significance which tends to infinity. Forvalues for which the sudden voltage change is large and sudden currentchange is small, or for which the sudden voltage change is small andsudden current change is large, the resultant significance is small, andtends to 0.

[0045] The significance, divided by 100 and limited to unity, results inthe weighting. The dynamic internal resistance Ri, which is obtainedfrom the quotient of the voltage change ΔU and current change ΔI, isweighted by means of this weighting factor.

[0046] A measured value with an expected error of less than 1% wouldthus be transferred immediately.

[0047] The rest of the calculation process takes account only of thosevalues of the dynamic internal resistance Ri for which one of thefollowing conditions is satisfied:

[0048] The weighting indicates a sufficiently small measurement error.

[0049] The sudden voltage change is very large.

[0050] The sudden current change is very large.

[0051] The alternative calculation with a large sudden voltage change orsudden current change means that it is also possible to calculate verysmall or large resistance values (but with a high error probability).

[0052] The new, filtered value of the dynamic internal resistance Ri isobtained from the old values and from the correspondingly weighted newvalue, using the formula:

RiFilt=RiFilt×(1−weighting flank)+Riflank×weighting flank

[0053] In the initial phase, the sum of all the previous weightingprocesses carried out is less than unity. In this situation, theweighting for the current flank is related to the sum of the existingweightings, and this relative value is used as the present flankweighting. The smaller the sum of the existing weightings and the higherthe weighting for the present flank, the greater the extent to whichthis present flank is thus taken into account.

[0054] In addition to the normal calculation, a “high-speed dynamicinternal resistance Ri-fast” is also calculated. This is composed ofonly the last 20 calculated values and thus has a considerably fasterdynamic response, and is less accurate, than the filtered dynamicinternal resistance Ri. In the event of major discrepancies between thefiltered dynamic internal resistance Ri and the high-speed dynamicinternal resistance Ri-fast, the high-speed dynamic internal resistanceRi-fast is used, provided that it is sufficiently stable and that one ofthe values (filtered or high-speed) is above a specific minimum value.If no values of the dynamic internal resistance Ri are determined for avery long time, the high-speed dynamic internal resistance Ri-fast isreset, in order to produce values more quickly when stimulated onceagain.

[0055] The validity is assessed in measuring the time which is requiredto calculate the high-speed dynamic internal resistance Ri-fast. If apredetermined time interval has passed here, the validity may be assumedto be no longer adequate.

[0056] A statement about the buffering effect of the battery 2 can bemade from the validity and from the determined dynamic internalresistance Ri.

[0057] There are two output variables for the statement about thebuffering effect of the battery 2:

[0058] 1. A first output signal A1, which always produces a value, evenwhen the stimulus is no longer sufficient. This value is required inorder to ensure that a statement is always produced about the bufferingeffect of the battery 2. If the stimulus is no longer sufficient, thisoutput signal remains at the last determined value. New values are notdetermined again until the validity is sufficient.

[0059] 2. A second output signal A2, which produces a value only whenthe validity is sufficient. If this is no longer the case, the outputsignal produces the value for “not available”.

[0060] The statement about the buffering effect of the battery isupdated whenever a new dynamic internal resistance Ri has beendetermined and the validity is sufficient.

[0061] The output variable is formed in two stages. Firstly, three areasare assessed independently of one another. The areas are then checked,and the output variables are formed.

[0062] There are a number of critical areas in which the dynamicinternal resistance Ri may move. These are assessed independently of oneanother on the same principle, with different parameters (thresholds andtimes). Each area has an associated upper and lower threshold as well asa minimum time for overshooting of the upper threshold and a minimumtime for undershooting of the lower threshold. In consequence, for verylow values (<20 mΩ), the dynamic internal resistance is calculated lessaccurately than, for example, at 80 mΩ. Furthermore, the criticalbattery state must be identified more quickly than, for example, duringexternal starting.

[0063] An upper threshold value Tho and a lower threshold value Thu aredefined as thresholds for the dynamic internal resistance. In addition,two time intervals To and Tu, respectively are defined, during which thethresholds must be exceeded by the dynamic internal resistance Ri beforethe state for the corresponding area changes.

[0064] When a valid value for the dynamic internal resistance Ri isdetermined for the first time after resetting, a value is in each caseassumed for the buffering effect. This is Ri-lower if the value is belowthe upper threshold value, otherwise it is Ri-higher. This procedure waschosen since, otherwise, no statement could be made for a relativelylong time when the determined dynamic internal resistance Ri is betweenthe two threshold values, and the battery state is normally improving.Furthermore, no message is produced in the better case.

[0065] A change in the corresponding range variable then occurs onlywhen:

[0066] The dynamic internal resistance Ri is below Thu for longer thanTu when the state is poor; the buffering effect is then set to Ri-lower.

[0067] The dynamic internal resistance Ri is above Tho for longer thanTo when the state is good; the buffering effect is then set toRi-higher.

[0068] Various thresholds and times are stated as the value range owingto the various battery sizes and technologies that are to be used, andthese are used to form individual range bits, as is shown in Table 1, sothat it is possible to distinguish between a threshold for a criticalvehicle power supply system state, a threshold for the initiation ofadditional measures, for example connection of a second battery (back-upbattery) bat 2, and a threshold for external starting fs. TABLE 1 Tho[mΩ] Thu [mΩ] To[s] Tu[s] Comments Ri-crit 220-130 140-90  0.5-3  0.3-3   Threshold for critical vehicle power supply system state Ri-bat290-20 50-15  3-20  3-20 Threshold for initiating additional measures,for example connection of a second battery Ri-fs 30-15 20-8  10-40 10-40Threshold for external starting

[0069] Once the individual range bits have been formed, these are usedtogether with the information about the validity in order to form theoutput variables A1 and A2.

[0070] The logic linking of these range bits is shown in Table 2 below:TABLE 2 Validity Ri-crit Ri-bat2 Ri-fs OK A1 A2 0 0 0 0 A1-io A2-nv 0 00 1 A1-io A2-io 0 0 1 0 A1-fs A2-nv 0 0 1 1 A1-fs A2-io 0 1 0 0 A1-bat2A2-nv 0 1 0 1 A1-bat2 A2-bat2 0 1 1 0 A1-bat2 A2-nv 0 1 1 1 A1-bat2A2-bat2 1 0 0 0 A1-crit A2-nv 1 0 0 1 A1-crit A2-crit 1 0 1 0 A1-critA2-nv 1 0 1 1 A1-crit A2-crit 1 1 0 0 A1-crit A2-nv 1 1 0 1 A1-critA2-crit 1 1 1 0 A1-crit A2-nv 1 1 1 1 A1-crit A2-crit

[0071] where A1-io is equivalent to the buffering effect beingsatisfactory, A1-fs is equivalent to the buffering effect in theexternal starting range, A1-bat2 is equivalent to the buffering effectin the range in which the second battery should be connected, A1-crit isequivalent to a critical buffering effect, A2-io is equivalent to asatisfactory buffering effect, A2-fs is equivalent to a buffering effectin the external starting range, A2-bat2 is equivalent to a bufferingeffect in the range in which the second battery should be connected,A2-crit is equivalent to a critical buffering effect, and A2-nv isequivalent to a value for the buffering effect not being available.

[0072] In order to provide sufficient stimulus for the process in phasesin which a valid determination of the dynamic internal resistance Ri isrequired, one preferred development of the invention provides for theheated rear windshield to be used, if required, in order to produceadditional flanks. This is done by deliberately operating the heatedrear windshield, although the following boundary conditions must besatisfied in this case, to ensure that a stimulus is provided by theheated rear windshield:

[0073] The instantaneously determined value of the dynamic internalresistance Ri must be greater than a specific value.

[0074] The generator must be active.

[0075] The instantaneous stimulus is not sufficient.

[0076] A minimum time Tmin must have passed since the end of the laststimulation pulse.

[0077] If these boundary conditions are satisfied, a drive device in themonitoring device first of all deactivates the heated rear windshield ifit is being operated. Once a delay time THSS off has elapsed, the heatedrear windshield is activated (again). A second delay time THSS on isthen allowed to pass, after which the heated rear windshield is returnedto normal control by the drive device.

[0078] This means that the dynamic internal resistance Ri can bedetermined reliably and easily even in a situation in which the normalstimulus is missing, so that a statement can be made without anyproblems about the buffering effect of the battery 2.

[0079] The following text describes various measurements on the battery2 with reference to diagrams, and explains their effects on thebuffering effect of the battery 2 in more detail. By way of example,FIG. 3 shows a diagram of a current/voltage characteristic of a chargedbattery 2. The upper curve shows the voltage U, and the lower curveshows the current I from the battery 2. The voltage ripple UW is verysmall despite the pulsed current change ΔI. The battery 2 thus has alargely good buffering effect. In this case, as described above, thevoltage ripple UW is monitored by the arrangement 1 for compliance withthe predetermined limit value G_(UW).

[0080] By way of example, FIG. 4 shows a current/voltage characteristicfor a measurement with a battery 2 which has been discharged to a majorextent, and no longer has any buffering effect. The voltage ripple UW(upper curve) is considerably greater than that of the measurement inFIG. 3, and this is thus an indication that a buffering effect of thebattery 2 is insufficient.

[0081] In addition to determination and detection as well as monitoringof the voltage ripple UW and/or of the current ripple IW, the behaviorof the vehicle power supply system voltage U and of the vehicle powersupply system current I, particularly when a load is connected, can alsobe used to assess the buffering effect of the battery 2, as alreadyexplained above.

[0082]FIGS. 5A and 5B show the voltage profile (top) and the currentprofile (bottom) while a load is switched on, for example the rearwindshield heating. The measurement was carried out without any battery2, that is to say there was no buffering effect from the battery 2.Since there is no battery 2, no battery current I flows either, and allthe current I must be provided from a generator 5. The sudden rise inthe current I in the vehicle power supply system 4 first of all resultsin the voltage U collapsing, although a regulator within the generatorregulates the voltage U to its nominal value once again within about 600ms.

[0083] The fact that the load has been switched on, in this case therear windshield heating, is in this case determined by means of thevoltage sensor 6 and the current sensor 10 or, alternatively, by meansof a signal via a databus. Once the load has been switched on, thevoltage dip as determined by the voltage sensor 6 and the current sensor10, and the current profile are assessed by means of the monitoring unit8, which is in the form of hardware circuitry and/or software. Thecurrent profile shows when the load is switched on, and the bufferingeffect is deduced from the associated voltage profile. If the voltage Ufalls below a predetermined limit value G_(U), for example of 12.5 V,then this indicates that the buffering effect of the battery 2 is nolonger sufficient.

[0084] The current change ΔI can also be used to estimate the currentdrawn by the load. If the current change ΔI is considerably less thanthe rated current of the load, then the battery 2 can no longer supplysufficient current I. In this case, the detection of the voltage U andof the current I is carried out continuously, in particular immediatelyafter the load is switched on, and within the regulation time of thegenerator 5. For test purposes, the load can be switched on and off twoor more times in order to ensure that the measurement does not have anyother load, switched on on a random basis, superimposed on it, with themeasurement providing incorrect statements.

[0085] As already explained in more detail above, the voltage change ΔUand the current change ΔI can be used to determine the dynamic internalresistance. As already stated above, the quotient ΔU/ΔI is determinedfollowing a significant current or voltage change, for example of ΔU=1Vor of ΔI=5 A, in the vehicle power supply system 4. In this case, themonitoring unit 8 monitors the quotient of the voltage change ΔU and thecurrent change ΔI for a predetermined limit value G_(R). The smaller thequotient of ΔU/ΔI, the better is the buffering effect of the battery 2.The monitoring therefore looks for the limit value G_(R), in particulara minimum value, being exceeded.

[0086]FIGS. 6A and 6B show the same measurement as that shown in FIGS.5A and 5B, but with a defective battery 2, that is to say a battery 2with only a limited buffering capability. The battery 2 is first of allcharged until the time t=10.25 s, after which the rear windshieldheating is switched on, and the current I then becomes negative. In thiscase, the battery 2 does not supply all of the current I that isrequired to operate the rear windshield heating. The sudden change inthe battery current I after the rear windshield heating or the load isswitched on is less than the actual current (rated current) of the rearwindshield heating. In consequence, the voltage U from the generator 5largely collapses, that is to say down to U=12.5 volts. The generator 5once again regulates the voltage U with an appropriate time constant. Inthis case, the monitoring unit 8 can use the high voltage ripple UW orthe reduced sudden current change and the major dip in the voltage as acriterion that the battery 2 has a poor buffering effect.

[0087]FIGS. 7A and 7B show the current/voltage characteristic for thesame measurement as that shown in FIGS. 6A and 6B, but with a battery 2having a better buffering capability than that battery. First of all,the battery 2 is charged until a time t=9.5 S, after which the rearwindshield heating is switched on and the current I becomes negative. Inthis case, more current I is drawn from the battery 2 for operation ofthe rear windshield heating than in the case shown in FIG. 6B. Thevoltage U from the generator 5 thus dips to a lesser extent, that is tosay down to U=13.25 volts. The generator 5 once again regulates thevoltage with the corresponding time constant. In this case as well, themonitoring unit 8 can use the high degree of voltage ripple UW or thereduced sudden current change and the major drop in voltage as acriterion for a poor buffering effect.

[0088] In contrast, FIGS. 8A and 8B show the current/voltagecharacteristic for the same measurement with a battery 2 with a fullbuffering capability. This clearly shows that the voltage ripple UW isconsiderably less than that of FIGS. 5A, 5B to 7A, 7B. The battery 2 isfirst of all charged until a time t=6.6 s, after which the rearwindshield heating is switched on. The current I becomes negative, andvirtually all of the current I for the rear windshield heating can bedrawn from the battery 2. In consequence, the voltage U from thegenerator dips by only about 150-200 mV. The monitoring unit 8 uses thereduced voltage ripple UW to determine that the battery 2 has a largelygood buffering effect.

[0089] FIGS. 9A-9C to 11A-11C show the current/voltage characteristic aswell as the characteristic for the dynamic internal resistance Ri of abattery 2 which has been deep-discharged and which has been thawed out.In FIGS. 9A to 9C, the battery 2 is still severely frozen and is notabsorbing any charge, that is to say the current I=0. In this state, themonitoring unit 8 monitors in particular the dynamic internal resistanceRi, which is very high, with Ri being greater than 1 Ω. The battery 2therefore has no buffering effect. FIGS. 10A to 10C show thecharacteristics for the same battery 2 about 18.6 h after it has thawedout. In this case, the battery 2 is drawing current I. The dynamicinternal resistance Ri has fallen to a value of between 250 and 350 mΩ.By monitoring the dynamic internal resistance Ri, the monitoring unit 8thus identifies a buffering effect, since the dynamic internalresistance Ri has fallen below the associated limit value G_(R). Oncethe battery has been charged for a period of 7 minutes, the dynamicinternal resistance Ri falls to a value below 250 mΩ. The associatedcharacteristics are shown in FIGS. 11A to 11C.

[0090]FIGS. 11A to 11C show the current/voltage characteristic and thecharacteristic for the dynamic internal resistance Ri for a fullycharged battery 2 with a maximum buffering effect. The dynamic internalresistance Ri has fallen to a value of 10 to 20 mΩ.

[0091] Depending on the nature and the configuration of the arrangement1, the voltage ripple UW or the voltage drop can be monitored separatelyor in a combined manner in order to determine the buffering effect ofthe battery 2. Furthermore, any other desired combinations of themonitoring of the current ripple IW, of the decreasing current I, of thevoltage drop U and/or of the dynamic internal resistance Ri can bemonitored for compliance with limit values G.

[0092] In summary, in order to identify the serviceability of a batteryin as simple and reliable a manner as possible, the present inventiondiscloses a method for determining the buffering effect of the battery(2) for providing a voltage (U) for a power supply system (4), inparticular for a vehicle, in which a voltage change and a current changeare detected cyclically, the dynamic internal resistance of the batteryis determined on the basis of the quotient of the voltage change and ofthe current change, the determined dynamic internal resistance ismonitored for exceeding a limit value which can be predetermined, and astatement is made about the buffering effect of the battery, and isoutput. In this case, the buffering effect of the battery is better thesmaller the quotient of the voltage change and current change.

1. A method for determining the buffering effect of a battery (2) forproviding a voltage (U) for a power supply system (4), in particular fora vehicle, characterized by the following steps: cyclic detection of anyvoltage change (AU) and of any current change (ΔI), determination of thedynamic internal resistance (Ri) of the battery (2) on the basis of thequotient of the voltage change (ΔU) and of the current change (ΔI),monitoring of the determined dynamic resistances (Ri) by means of amonitoring device (8) in order to determine whether a limit value(G_(R)) which can be predetermined has been exceeded, and making astatement on the buffering effect of the battery (2) and outputting thisstatement relating to the buffering effect of the battery (2), with thebuffering effect of the battery (2) being better, the smaller thequotient of the voltage change (ΔU) and current change (ΔI).
 2. Themethod as claimed in claim 1, characterized in that the limit value(G_(R)) which can be predetermined is a minimum value or maximum value.3. The method as claimed in claim 1 or 2, characterized in that thevalues of the detected voltage change and current change are filteredbefore the determination of the dynamic internal resistance (Ri) and aresearched by means of the monitoring device (8) for extreme values, thatis to say relative minima and relative maxima, at which the dynamicinternal resistance (Ri) is determined, when predetermined boundaryconditions are satisfied.
 4. The method as claimed in claim 1 or 2,characterized in that the detected current and/or voltage changes areprocessed by two or more parallel filters with different time constants,and the comparison between the output signals of the filters is used asan enable signal for the calculation of the internal resistance.
 5. Themethod as claimed in claim 4, characterized in that decision criteriafor the comparison of the output signals from the filters are dependenton the status of the vehicle power supply system (charging of thebattery, discharging of the battery, transitional state) and/or on themagnitude of the current or voltage change.
 6. The method as claimed inclaim 3, characterized in that the relative error in the resistancedetermination is used to derive a significance which, divided by 100 andlimited to unity, provides a weighting factor for the dynamic internalresistance (Ri).
 7. The method as claimed in claim 6, characterized inthat a new filtered value of the dynamic internal resistance (Ri) isdetermined from old values, which have already been stored in the past,for the dynamic internal resistance (Ri) and the new determined value,weighted in accordance with the weighting factor, for the dynamicinternal resistance (Ri) for a limit value which has been found.
 8. Themethod as claimed in claim 6, characterized in that, for rapidassessment of the dynamic internal resistance, only the last 20calculated values of the dynamic internal resistance are evaluated. 9.The method as claimed in claim 7, characterized in that the statementabout the buffering effect of the battery (2) is updated whenever a newdynamic internal resistance (Ri) has been determined.
 10. The method asclaimed in one of the preceding claims 1 to 9, characterized in that anupper and a lower threshold value (Tho, Thu) are defined for the dynamicinternal resistance (Ri), as well as two time intervals (To, Tu) duringwhich the thresholds must be overshot (To) or undershot (Tu) by thedynamic internal resistance (Ri) for any statement about the bufferingeffect of the battery (2) to be output.
 11. The method as claimed inclaim 10, characterized in that the threshold values and time intervalsare in each case set differently for different ranges, so that it ispossible to distinguish between a threshold and a time interval for acritical vehicle power supply system state, a threshold and a timeinterval for an additional measure, for example connection of a secondbattery, and a threshold and a time interval for an external start. 12.The method as claimed in one of the preceding claims 1 to 11,characterized in that, if one stimulus is not sufficient for validdetermination of the dynamic internal resistance (Ri), additional limitvalues are produced by deliberately driving a load.
 13. The method asclaimed in claim 12, characterized in that a heated rear windshield isoperated as a load.
 14. The method as claimed in one of the precedingclaims 1 to 13, characterized by the further step of outputting awarning message (M) if the limit value (G_(R)) is exceeded.
 15. Anarrangement for determining the buffering effect of a battery (2) forproviding a voltage (U) for a power supply system (4), in particular fora vehicle, characterized in that means (6, 10) are provided fordetecting and determining the voltage and the current, and themonitoring unit (8) is provided for determining the dynamic internalresistance of the battery (2) on the basis of the quotient of thevoltage change (ΔU) and the current change (ΔI), monitoring of thedetermined dynamic internal resistance (Ri) for exceeding a limit(G_(R)) which can be predetermined, and making a statement for thebuffering effect of the battery (2), and outputting this statement aboutthe buffering effect of the battery (2).
 16. the arrangement as claimedin claim 15, characterized in that the monitoring device (8) has adevice for deliberately driving a load when no adequate stimulus isavailable for determination of the dynamic internal resistance (Ri).